Variable resistor



Aug. 22, 1944. 7 F, I F s 2,356,512

VARIABLE RESISTOR.

. Filed Sept. 4; 1940 f Fi .4

" mmvrozz Friedrich Gieffers ATTORNEK Patented Aug. 22, 1944 UNITED STATES PATENT OFFICE VARIABLE RESISTOR Friedrich Gieffers, Berlin-Spandau, Germany; vested in the Alien Property Custodian 7 Claims.

This invention relates to variable resistors having a solid resistance body and adapted to carry very high loads, and more particularly to variable resistors for controlling power circuits. Such variable resistors involve the risk that the current flow assumes a very high density locally Within the resistance body so that certain portions, particles or the like volumetric elements of the body are overloaded and deteriorated. When carrying out switching operations in power circuits, such resistors must be capable of varying the magnitude of their resistance within a very short time and therefore must also withstand considerable mechanical stresses. Any thermal overstress of the resistance body or the contact cooperating therewith reduces at the same time the mechanical strength of the variable resister and may render it unsuitable for the above purposes.

The main object of the invention is to provide a variable resistor for heavy duty, in particular for effecting or preparing switching operations in highly loaded power circuits, which is free of the above-mentioned drawbacks, i. e. protected against deterioration due to overloading of local portions or particles of the resistance body. Other objects will become apparent from the following description.

According to the invention, the eifective resistance of a solid resistance body is varied by means of a slide contact or the like control device while simultaneously displacing the path of the current flow within the body. This simultaneous displacement of the current path is so effected that particularly the zone of highest current density, i. e. the core of the current path, passes successively through different elements or particles of the resistance body so that the elements, particles, or sections of the resistance body carry the maximum load only temporarily as compared with the total duration of a resistance variation.

The control or resistance variation, in particular for controlling power circuits, is preferably effected within a fraction of a second. Hence the parts of the resistance body carrying the current core are subjected to maximum electrical stresses only during a portion of the short duration of control. Consequently, the elements of the resistance body carrying the current core are exchanged in the shortest possible time by other elements so that the points of highest temperature move rapidly through the resistance body so that no undesirable accumulation of heat can occur in any local part of the resistance body.

As a result, the resistance material is kept in good condition and remains operative for an increased length of time even if used for heavy duty control in high power circuits.

The above-described shifting of the current path, in particular its core of maximum current density, can be accomplished, according to the invention, by causing the inlet point as well as the outlet point of the current path to move along the resistance body while effecting the resistance variation. While in the known variable resistors or rheostats a current terminal of the resistance body is stationary and only the other terminal changes its position, the invention in one of its aspects provides that both terminals or contacts for supplying current to the resistance body proper move so that the entire current path is shifted relatively to the resistance body. However, the invention provides also other possibilities for displacing both end points of the current path while varying its effective resistance, as will be set forth in the following in connection with the embodiments diagrammatically illustrated in the drawing.

Figs. 1, 2 and 3 show three different resistance devices in which a single resistance body proper cooperates with two movable contacts, the resistance body being shown in a longitudinal section.

Figs. 4 and 5 illustrate a fourth embodiment in two diflerent operating positions respectively. This embodiment contains two resistance bodies, shown in longitudinal section, which are movable relative to each other.

Fig. 6 represents a modified form of a resistance body applicable in devices of the type shown in Figs. 4 and 5; and

Fig. 7 is a cross section through a further example having two resistors and a rotary contact.

The variable resistance device shown in Fig. 1 contains an elongated rod or plate-shaped resistance body w and two slide contacts s1 and 32 which form the terminals for passing the electric current through the resistor. In order to vary the effective resistance, the two contacts are shifted in the direction indicated by the arrows to the position shown in dash lines so that the middle portion of the current path 0 is shifted Within the resistance body in its longitudinal direction. The same eiTect occurs when the contacts are kept stationary while the resistance body is shifted relative thereto. The change in resistance is brought about by the fact that the local specific resistance of the resistance body w increases in its longitudinal direction so that the two contacts, while maintaining a constant distance relative to each other, are shifted to a portion of the body in which the current path has a different resistance. In the case of a homogeneous resistance material, the resistance between the contacts may be varied by moving both contacts with different speeds. If, for instance, in the end position the contacts are spaced from one another more than in their initial position, the effective resistance between the contacts is greater. According to the invention, the two contacts must in this case also be displaced to such an extent that the effective current path is shifted to another portion of the resistance body.

It is particularly advantageous, if the movement of the current contacts for varying the resistance magnitude and their motion serving to displace the current core, take place in different directions. For instance, one or both of the contacts s1 and s2 in the arrangement shown in 1 may be displaced during the control operation in the plane of the drawing, while they are shifted at the same time over the surface of the resistance body to perpendicularly to the plane of the drawing. Also in this case the portion of the current path imposing maximum stress on the resistance body, i. e. the portion referred to above as the current core, tends to maintain the shortest possible distance between the two contacts. This current core is therefore shifted according to the invention during the control operation to other points of the resistance body.

As shown in Fig. 2, the two contacts s1, 52 may also lie at different sides of the resistance body w. This has the advantag that the surfaces over which the contacts slide are better cooled. and that both contacts may be moved in the direction of the line connecting the same without over1ap ping of the surfaces over which the contacts slide. Also in the arrangement shown in Fig. 2 is it possible to shift the current core in the longitudinal direction of its middle portion as well as perpendicularly thereto with respect to th resistance body.

According to Fig. 3 the transverse displacement of the current core is effected in the direction of the contact movement efiect-ing the change in resistance. To this end, the two contacts 81 and .82 are arranged directly opposite each other on a resistance body whose specific resistance varies continuously or step by step. In this case, the current path between the contacts runs across the resistance body. When the contacts are displaced to points having another specific resistance as indicated in dash lines, the current core moves at the same time perpendicularly to its longitudinal direction without substantially varying its lengt Instead of one of the movable contacts, for instance contact 32 in Fig. 3, a stationary contact or terminal may be arranged on the rod or plateshaped resistance body at the side away from the movable contact. This stationary contact must extend over a considerable portion of the length of the resistance body, as will be understood from the embodiments described in the following.

According to Fig. l, two rod or plate-shaped resistance bodies .01 and wz, having their resistance increasing from one to the other end of each body, slide over each other. Each of the resistance bodies has a contact or terminal or or as arranged on its longitudinal sur ace paral el to the slide surface so that the contact extends over the entire length of the body. The contacts or and a2 consist of metal layers which are integral with metal contacts on and mg respectively lying at the end of the corresponding resistance body where the specific resistance is smallest. In particular cases non-metallic parts having a sufficiently small specific resistance may be employed instead of these metal parts. In the position shown in Fig. 4 the parts m1 and 1212 are in engagement with each other. The connection is therefore practically without resistance. If the two bodies are shifted with respect to each other in the direction of the arrows, sections of higher specific resistance are inserted one after the other into the core current path until, in the end position shown in Fig. 5, the sections of the maximum specific resistance lie upon one another. During the control, the current paths run substantially perpendicularly to the contact surface of the two resistance bodies and new sections and elements of the resistance bodies come continuously into the zone of the maximum current density.

In the control positions of the greatest resistance the current load of the resistance body is small and non-detrimental, and in this phase of control the stressed volume elements need no longer change their position. Consequently, it sufiices if the contacts or and as, as shown in Fig. 6, extend only over a considerable portion of length of the resistance bodies 'ZUl and 102. The same applies to the forms of the invention shown in Figs. 4 and 5.

According to another feature of the invention, the proper shifting of the current core in the resistance body is facilitated by subdividing the movable contact. Such subdivided contact has several metal layers separated by insulating layers or layers of greater specific resistance which extend substantially perpendicular to the contact surface of the resistance body. In Fig. 7 is shown, for instance, a laminated contact s with insulating layers 1'. This form of the invention prevents a crowding of the current paths at the edge of the contact, particularly in one or both end positions of the contact. The resistance bodies an and 204 are arcuate and come into engagement with the current contact s at opposite cylindrical surfaces. The specific resistance of the bodies wa and 1114 increases from the end provided with th lateral metal portions ma and mi of the stationary terminals or contacts a3 and (14 respectively towards the other end. The current contact s is associated with insulating pieces i1 and i2 to form a cylindrical body. Fig. 7 shows the device in the position of smallest resistance. When rotating the cylindrical contact assembly in the clockwise direction, the resistance is increased up to a maximum value while the current path, and in particular the core of highest current density of the path, is displaced tangentially so as to traverse successively different resistance elements or particles of each resistance body.

What is claimed is:

1. A variable electric resistance device, in particular for high-power duty, comprising a solid resistance body and a contact device for varying the effective resistance of said body, said device comprising two contacts engaging said body at opposit sides to pass current through said body along a current path extending across said body, one of said contacts being firmly attached to said body and having a contact surface extending over the major portion of the appertainingcontact surface of said body, said other contact being slidable along said contact surface of said body, and means for moving said latter contact relative to said body to vary the resistance of the current path extending between said contacts through said body while displacing the location of said path within said body.

2. A variable electric resistance device, in particular for high-power duty, comprising a resistance body having a specific resistance increasing from one to the other end of the body, two contacts engaging said body at substantially opposite sides to pass current across said body, one of said contacts being firmly connected with said body and extending over a substantial portion of the appertaining contact surface of said body, said other contact being movable relative to said body to vary the effective resistance of the body while displacing the current path within said body.

3. A variable electric resistance device, in particular for high-power duty, comprising a resistance body having a specific resistance increasing from one to the other end of the body, a contact covering a substantial portion of a surface of said body along the direction in which said specific resistance varies, and slidable contact means arranged at another surface of said body substantially opposite to said contact, said contact means being displaceable relative to said body for varying the effective resistance of the current path extending from said contact means across said body to said contact, whereby the end point at said contact of the current cor of highest current density is shifted along said contact when varying the effective resistance.

4. A variable electric resistance device, in particular for high-power duty, comprising an elongated resistance body having a specific resistance increasing in the longitudinal direction of said body, a contact covering a substantial portion of a longitudinal surface of said body and having a free contact surface flush with the opposite longitudinal surface of said body and adjacent to the end portion of smallest specific resistance of said body, and contact means movably arranged at said latter longitudinal surface of said body so as to selectively engage said free contact surface and said latter surface of said body, whereby the current path traversing said body between said contact means and said, contact is displaced in the longitudinal direction of said body when moving said contact means from said free contact surface of said contact towards the other end of said body.

5. A variable electric resistance device, in particular for high-power duty, comprising two resistance structures each including an elongated resistance body having a specific resistance increasing in the longitudinal direction of said body, a contact covering a substantial portion of a longitudinal surface of said body and having a free contact surface flush with the opposite longitudinal surface of said body and adjacent to the end portion of smallest specific resistance of said body, and a movable contact arranged between said two resistance structures and interconnecting said two free contact surfaces when placed in the position corresponding to the smallest resistance while successively interconnecting portions of increasing specific resistance of said opposite longitudinal surfaces of said bodies when moved into a position corresponding to a higher resistance.

6. A variable electric resistance device, in particular for high-power duty, comprising two resistance bodies each having a cylindrical contact surface arranged opposite the contact surface of the other body and each having also a contact element forming a current supply terminal and arranged so as to cover a substantial portion of the surface of said body opposite to said cylindrical contact surface, each of said bodies having a cross sectional resistance increasing in a tangential direction from one end towards the other end of the body, and a rotary contact arranged between said bodies coaxially with said cylindrical surfaces and engaging part of each cylindrical surface so as to interconnect parts of said bodies of lower cross sectional resistance when in one position and parts of higher cross sectional resistance when rotated for adjusting a higher resistance.

7. A variable electric resistance device, in particular for high-power duty, comprising a resistance body having a specific resistance increasing from one to the other end of the body, a contact covering a substantial portion of a surface of said body along the direction in which said specific resistance varies and a slide contact engaging another surface of said body opposite to said first surface, said slide contact being subdivided in perpendicular direction relative to said opposite surface and comprising a plurality of contact elements and elements of higher resistance interposed between said contact elements.

FRIEDRICH GIEFFERS. 

